A couple of years ago, while at another rubber stamp show, I saw a demo at the Just For Fun booth where they used those same postage grid stamps with a template and used greeting cards. I love to recycle and re-use so I purchased the templates to match the stamps I already had. Just For Fun has many different stamps and templates to chose from and I also ended up with another postage grid stamp and template.
This technique is very easy. Choose a template that works with your already-used card. Not all cards work for this technique and you have to play around with the templates, moving them here and there to find the best fit.
Draw around the openings in the template with a pencil. Cut on the lines (I used a pair of stamp edge decorative scissors) being careful to keep each piece in the right order. It's difficult to reposition all the little rectangles if they get mixed up. Trust me. Stamp the matching postage grid stamp on background paper, adhere the little rectangles in the correct place and trim around the outer edges. Adhere to a card.
This is a great way to re-use those greeting cards. It's easy and once you get going you won't be able to stop.
I might even get around to using those stamps for the original purpose some day!
Happy inky fingers!